A fallacy? Sharing code between client and server is easier when the two are in the same language than when the two are in a different language. How is that flawed reasoning?

One example: you can run the same input validation in the client as you do in the server if you can easily share code between the two.

But there is way more. Here's an example of an application that shares all of its data models between client and server: http://tilemill.com/docs/models.html